A scorotron charger including a grid is known as a corona charger that charges a photosensitive member, which is a member to be charged. There are basically two types of grids. One type is a wire grid that includes wires arranged in an opening in a housing of a corona charger so as to extend in a longitudinal direction. The other type is an etching grid produced by forming many holes in a thin plate by an etching process.
The etching grid covers a larger area of the opening in the housing (has a smaller opening ratio) than the wire grid, and is therefore advantageous in that the potential of a photosensitive member can be easily controlled to a target potential. However, the etching grid more easily allows discharge products generated in a discharge process to adhere thereto than the wire grid.
The discharge products that have adhered to the etching grid (hereinafter referred to as grid) accelerate oxidation of the grid, and rusted parts of the grid have charging characteristics different from those of the other parts. This leads to non-uniform charging. PTL 1 discloses a structure for suppressing non-uniform charging by forming a protective layer, which mainly contains carbon atoms in an SP3 structure, on a surface of a base member of the grid and increasing corrosiveness against the discharge products. PTL 1 also discloses a structure in which the thickness of a protective layer that covers a front surface of the grid that faces a discharge electrode is greater than the thickness of a protective layer that covers a back surface of the grid since the surface of the grid that faces the discharge electrode is easily corroded by the discharge products.
The discharge products that have passed through the grid adhere to the photosensitive member. When the discharge products that have adhered to the photosensitive member absorb moisture, an image defect called “image deletion” occurs. Accordingly, PTL 2 discloses a structure for reducing adhesion of discharge products to a photosensitive member by covering an opening formed in a housing of a corona charger with a shutter.
However, in the case where the opening formed in the housing is covered with the shutter to reduce adhesion of the discharge products to the photosensitive member when the image forming operation is not performed as in PTL 2, the discharge products accumulate on the shutter. It has been found through studies conducted by the inventors that the discharge products that accumulate on the shutter accelerate corrosion of the protective layer on the discharge-electrode side of the grid.